Hot water from cold tap

[hycslim]

Hi

I have a raritan hot water tank on our Sabre 402. When any cold tap is run for a minute or so hot water starts to come out. As soon as the pressure pump kicks in the water will go cold again. Is there a malfunctioning one way pressure switch that is allowing this to happen?

Thanks

Allan

[xeon_tsd]

maybe you are in Texas?

Ok so that didn't help.. sorry.

[Stu Jackson]

Could be that the hot and cold water lines are run for some distance one next to the other. Are they?

[Irish rambler]

Insulate the cold water lines as they pass through the engine room.

[CaptTom]

I've got the same thing, and it's not just that the cold runs by a hot pipe or through a hot area. I can only guess that there's a check valve missing or stuck open and the pressure in the water heater is greater than the rest of the cold side of the system, at least until the pump kicks on.

[Irish rambler]

As a matter of interest do you have a vent/header tank on your system. It may be it can't vent the expanding hot liquid, having said that it would explode/split a connection if it weren't vented. Unfortunately unless I saw the system I can't make constructive comments. Sorry my friend.

[hycslim]

Thanks for the comments so far. It has nothing to do with lines running close to engine. Last year the system worked fine, only started this year once the boat went back in the water. There aren't any leaks but I have to think that maybe something froze over the winter.

Allan

[Wakuu]

I have a check valve on the cold water inlet side of my water heater. Prevents temp creep, which is what it sounds like you have.

[lateral]

Yep. Non-return valve cold water inlet.

[Suijin]

Originally Posted by Wakuu

I have a check valve on the cold water inlet side of my water heater. Prevents temp creep, which is what it sounds like you have.

I have the same issue and I'm betting dollars to donuts this is what it is. Thanks for the info.

Another trip to the chandlery *sigh*

Alex

[sailorchic34]

Two ways this an be happening. First is from expanding water, where the expanding water is pushed back towards the accumulator / expansion tank.

Second way is if the water heater is below the faucets. Then you can get gravity circulation / thermal siphon in either the hot or cold water lines. This as hot water is less dense then cold water and will tend to rise in either the cold or hot system (or both) depending on configuration of the tank and piping.

For the first a check valve will stop the back flow. However if a check valve is added then you need a thermal expansion tank (same think as an accumulator) installed after the Check valve or your T&P relief will pop off during every heating cycle

If you have a accumulator on the hot water system already, it may be water logged and may need to be drained and re pressurized.

For the second a check valve or heat trap (a u bend about 12" high) will prevent thermal siphon.

[Suijin]

Originally Posted by sailorchic34

For the first a check valve will stop the back flow. However if a check valve is added then you need a thermal expansion tank (same think as an accumulator) installed after the Check valve or your T&P relief will pop off during every heating cycle

I have a new "on demand" pressure pump. If I install a check valve at the inlet to the water heater, do you know if that going to cause a problem regarding how the pump senses pressure in the line?

[Sailmonkey]

Originally Posted by Suijin

I have a new "on demand" pressure pump. If I install a check valve at the inlet to the water heater, do you know if that going to cause a problem regarding how the pump senses pressure in the line?

A check valve will allow the pump to sense a drop in pressure, but not allow the expanded hot water to feed back into the cold line.

[sailorchic34]

As SM said the check valve will not effect the pump at all. It will prevent thermal expansion, so a expansion tank installed after the check valve will keep the T&P relief from popping off every heat cycle of the water heater.

[CaptTom]

Wouldn't the check valve be on the cold (inlet) side? I'm thinking the rest of the hot water plumbing would be able to absorb the expansion from the heating cycle, but just a wild guess.